In many operations, especially in the region of the head, problems may arise in terms of orientation during the operation because of the individual anatomic variations. There are numerous operations in which increased risk can be observed solely because of the problem of comprehensive, accurate orientation during the operation.
Continuous information on the position of surgical instruments in a given body part, especially knowledge of the spatial distance from delicate structures, such as blood vessels, lymph vessels, nerves, etc., increases the safety during operation. If it were possible to record and store this information, i.e., to make it reproducible, it would be possible to check the result of the operation after it has been completed. In the case of a failure arising through no fault of the surgeon, it would thus be possible to effectively head off unjustified malpractice suits.
Conventional X-ray pictures, computer tomograms, and/or, in exceptional cases, also intraoperative fluoroscopy have been used and applied so far to make orientation in the human body possible. Bony structures are primarily visualized in the X-rays. Using the more condensed information provided by computer tomograms for planning surgery has therefore been common practice. The application of the X-ray findings to the surgical procedure is accomplished by the surgeon. He checks visually the exact position of the surgical instrument intraoperatively. The surgical field is also occasionally measured or transilluminated. The latter is associated with all the disadvantages of the conventional X-ray technique and higher radiation load for the patient and the surgeon. In the case of intraoperative lateral fluoroscopy, another major disadvantage is the fact that the spatial relations in the body area to be operated on can only be represented by the image in the superimposed form. An extremely great wealth of experience is required in order to infer even approximately the actual spatial relations.
However, this does not provide continuous, reliable information on the position of the surgical instrument in relation to the position of the disease. As an alternative to the conventional methods, it is now possible to use computer-aided position information.
In neurosurgery, stereotactic operations are performed using a localization frame and an instrument holder. Such devices have been known, e.g., from West German Offenlegungsschrift (preliminary published patent application) No. 32,05,085, U.S. Pat. No. 4,465,069, EP-A-0,207,452, EP-A-0,018,166, and West German Offenlegungsschrift No. DE-OS 32,05,915. A specific V-shaped frame has also become known from U.S. Pat. No. 4,583,538, but it has been designed for and adapted to corresponding operations in the thorax rather than for neurosurgery.
Stereotactic surgery is part of neurosurgery and pertains to a class of operations in which probes, e.g., cannulae, needles, clamps, or electrodes, are to be applied to areas of the brain or other hidden anatomic targets, which are not visible from the outside. The stereotactic frame is used as a kind of "guiding device", which is used in human neurosurgery in order to guide an instrument to a special point within the brain through a small opening in the cranium by radiographic or other methods of visualization of reference points. The instrument must be brought to an exact, predetermined point as accurately as possible. Consequently, if the frame or the device is placed on the cranium, the probe can be moved forward to any given topographic point within the skull. The exact point is subsequently calculated from the distance determined and the direction between the reference point observed and the desired target in relation to the system of coordinates of the stereotactic device. By linearly moving forward the instrument, which is positioned accurately over the instrument holder held in said frame, a specimen is subsequently taken from the desired point, a local lesion is produced, or radioactive material is implanted.
Such methods were further developed in order to automate them as much as possible or in order to use, e.g., a laser coagulator. Punctiform lesions can be produced according to a plan prepared on the basis of computed tomograms. These known processes and devices also require the use of a frame rigidly adjusted to the head. It must also be borne in mind that accurate location of the frame can be achieved only by screwing at least three screws firmly into the cranial bone.
Contactless, i.e., frameless measurement for obtaining computer-aided position information for an instrument has also been known from a paper published in the journal, Neurosurgery, Volume 65, October 1986, pp. 445 ff. According to this process, the exact position of a surgical microscope is determined via three acoustic signal transmitters using air gaps and a total of four receivers. In addition, the computed tomograms previously entered into the memory can be projected into the focal plane of the surgical microscope in order to provide adequate assistance during the operation.
However, as was also mentioned in the previous publication, this process is essentially a stereotactic surgical system which operates only point by point and in which the working point is approached linearly, and, in addition, it can be used essentially only in the region of the cerebral cranium, but not in the bony cranium. This may also be due to the fact that the accuracy specified, exceeding 2 mm, is insufficient.
Furthermore, none of the known processes offer the possibility of documenting, in images, the course and the result of the surgical operation for subsequent checking, or no provisions are made for such documentation in the known processes.